1. Field of the Invention
This invention relates to a process for producing pure magnesium by molten salt electrolysis of magnesium chloride. More particularly, it relates to an efficient process for the electrolytic production of magnesium in which a process for the preparation of magnesium chloride which is capable of being directly subjected to electrolysis is combined with the molten salt electrolysis process.
2. Description of the Prior Art
Magnesium (Mg) is the lightest of the commonly-used metals, and it finds a wide variety of applications, including as an alloying element with aluminum, an inoculant in the manufacture of ductile cast iron, and a reducing agent in the production of titanium from titanium tetrachloride. The consumption of magnesium is still increasing.
There are two methods which are employed in the commercial production of magnesium: the thermal reduction method in which magnesium oxide (MgO) is reduced with ferrosilicon, and the electrolytic method in which magnesium chloride (MgCl.sub.2) is electrlyzed in a molten state. At present, more than 70% of magnesium is produced by the electrolytic method (C. L. Mantell, "Industrial Electrochemistry", McGraw-Hill, 1950).
Magnesium chloride for use in the electrolytic production of magnesium has been prepared by the following methods:
(1) hydrous magnesium chloride (MgCl.sub.2.nH.sub.2)) is dehydrated by heating with ammonium chloride;
(2) carnallite (MgCl.sub.2.KCl.6H.sub.2 O) is decomposed and dehydrated by heating; and
(3) hydrous magnesium chloride (MgCl.sub.2.nH.sub.2 O) is incompletely dehydrated by dissolving it in hydrochloric acid followed by evaporation and concentration of the solution until the hydrous salt has a value for "n" in the range of 1.25-2, and it is used in the electrolysis as such (Dow method).
the above methods (1) and (2) require a great amount of energy for dehydration by heating. In addition, according to method (2), potassium chloride (KCl) formed by decomposition of carnallite is built up in the electrolytic cell and must be removed periodically.
According to the Dow method &lt;method (3)&gt;, since water which is present in the incompletely dehydrated salt is also electrolyzed during the molten salt electrolysis, the consumption of the graphite anode is severe and it is necessary to use a special electrolytic cell. Another disadvantage is that the gas generated in the cell has a low concentration of chlorine so that it is difficult to reuse the gas in the preparation of magnesium chloride. Furthermore, a sludge composed mainly of MgO is accumulated on the bottom of the electrolytic cell during electrolysis.
Generally, in electrowinning of a metal, when the electrolytic bath is contaminated with other metals which are nobler than the target metal to be won, the contaminant metals are deposited on the cathode prior to or simultaneously with the target metal, thereby decreasing the purity and yield of the target metal.
For example, in the electrolytic production of magnesium, the molten salt bath is frequently contaminated with iron and manganese which are nobler than magnesium, and these contaminant metals are deposited on the cathode, thereby decreasing the purity of the magnesium product. The magnesium metal deposited on the cathode is usually collected after it floats on the surface of the molten salt. If a large amount of iron is deposited along with magnesium, the resulting contaminated magnesium has a specific gravity greater than that of the molten salt and will sink to the bottom of the electrolytic cell, thereby decreasing the yield of magnesium collected by flotation.
Iron and manganese ions have more than one valence to form redox systems as shown by the following equations: EQU Fe.sup.3+ +e.sup.- .fwdarw.Fe.sup.2+ ( 1) EQU Mn.sup.4+ +2 e.sup.- .fwdarw.Mn.sup.2+ ( 2)
Therefore, the lower valence ions of a contaminant metal, e.g., Fe.sup.2+, which are formed by reduction on the cathode move toward the anode and are oxidized thereon into the higher valence ions (Fe.sup.3+). Thus, ions of these contaminant metals move back and forth between the electrodes to perform oxidation and reduction repeatedly, leading to wasteful consumption of electric power which decreases the current efficiency.
In solution electrolysis, an ion-exchange membrane or other diaphragm is usually located between the electrodes in order to prevent large impurity ions from moving across the diaphragm. In molten salt electrolysis, however, a diaphragm is usually not used because a suitable diaphragm material which can withstand the high-temperature and corrosive environment of the molten salt is not readily available.
Accordingly, in the production of a metal by molten salt electrolysis, it is highly advantageous that the content in the cell bath of metals which are nobler than the metal to be produced be minimized in order to improve the purity and yield of the metal and current efficiency. Thus, it is desirable that such nobler metals be previously removed from the molten salt to be subjected to electrolysis.